In a grounded cathode amplifier, the gain can never exceed the mu of the triode. But in a cascode, this limitation disappears. For example a 12AU7 based cascode circuit can have a final gain of 260, if the Ra is large enough (1 meg). |
Before going horizontal, let us do a little mind stretching. Moving the ground reference point to the top of the circuit leaves the circuit functionally unchanged in terms of gain, output impedance, and PSRR, but it does alter the voltage relationships, as the grid is now at -200 volts. And both load resistors remain AC grounded at one end, which is a necessary condition for this circuit to work. |
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Where else can we move the ground reference point while still keeping to a single power supply? Two possibilities present themselves: either slide both the plate and cathode resistors together below or above the triode and connect the ground to the midpoint. |
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In the circuit above, the outputs are taken at the ends of the both load resistors. But how are two phases possible? As the triode conducts lesser or greater current, the resistors experience lesser or greater voltage across their leads, much like an accordion contracting and expanding. |
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